Conventional optical tracking systems and their image generating devices generally include a pair of cameras that record individual image points. These image points, which typically comprise positional markers and/or natural landmarks, are used to detect information on a position (e.g., the spatial position of the image points) from a stereoscopic image obtained from the cameras. Thus, there are existing systems that include an image processing unit within the camera system. These existing systems can provide spatial coordinates of detected image points, which then can be provided as an output signal. In principle, however, the images can also be processed externally.
Examples of generic tracking systems that perform optical three-dimensional surveying may be found in the following documents, the contents of which are hereby incorporated by reference in their entirety: DE 10 056 186 A1; U.S. Pat. No. 5,831,735; U.S. Pat. No. 6,493,095 B1; U.S. Pat. No. 6,437,820 B1; U.S. Pat. No. 5,828,770 and WO 00/16 121.
Within the medical field, optical tracking systems, for example, are presented in the following documents, the contents of which are hereby incorporated by reference in their entirety: U.S. Pat. No. 6,351,659 B1 or DE 19 639 615 C2; U.S. Pat. No. 6,484,049 B1 or U.S. Pat. No. 6,490,475 B1 (fluoroscopic tracking system) and U.S. Pat. No. 5,921,992.
Conventional tracking systems, which use camera systems having a predetermined image resolution, have the disadvantage that their position determining accuracy is substantially dependent on the resolution of the image generating devices. If relatively small markers or landmarks are to be recorded and their position determined, or in the case of markers and/or landmarks that are relatively far away from the image generating device, the accuracy of conventional image generating devices quickly reaches its limits. For example, FIGS. 4A-4C show how an image point P would be positionally detected on an 8×8 sensor 2 of a conventional image generating device. It is noted that the size of the image points (e.g. a marker) on the sensor 2 can be much smaller than a pixel 4 of the sensor 2, which in this conventional scenario results only in a maximum positional accuracy of one pixel.
More specifically, FIGS. 4A-4C illustrate an image point P spanning four pixels 4 of the sensor 2 and, therefore, exposing the four pixels 4 with different intensities within its projection P′ (FIG. 4B). Interpolating the “brightness” (e.g., the developing or illuminating intensity) of the group of pixels results in a sub-pixel resolution that ascertains a “focus” of the projection P′, which is approximately in the area of the actual image point P. Thus, the position of the point P can be calculated to be approximately at the point P″ as shown in FIG. 4C.
Such sub-pixel resolution is only possible when the image point P actually exposes more than one pixel 4. In the worst case scenario (i.e., wherein the image point P is within a single pixel), the image point position only can be determined as being within that pixel 4. Thus, in the case of small markers or image points having a size less than one pixel 4, a significant error can arise.
Accordingly, the accuracy of the conventional optical system can be determined by the resolution of the sensor 2 and the dynamics and linearity of the sensor pixel 4 (and in the 3D scenario, by the homogeneity of the marker). In the case of medical tracking systems or other tracking systems, which in many cases depend on accuracies of fractions of millimeters, an accuracy error of one pixel typically cannot be tolerated. Hence, attempts have been made to make markers as large as possible, thereby enabling one to determine the marker position by averaging a number of exposed pixels 4.
In other words, the size of markers and their maximum distance from the image generating device can be determined by a resolution and a reproduction scale of the lenses. While the size of image points could be increased simply by using larger markers, larger markers would incur significant disadvantages related to handling, cost and weight of the markers.
Furthermore, several other preconditions should be fulfilled in conventional systems in order to determine the position of such image points, namely:                pixel exposure should be triggered only by a single object, otherwise the weighting of grey scales for the sub-pixel resolution can be off;        the focus should be the interpolated center for the brightness (this is not the case if a marker is partially hidden, is not round or is tarnished, or if the reflecting surface is not projected with a uniform brightness);        if viewed from two different angles, a round marker should produce the same image on two sensors (this is likewise not the case if the marker is not round, is tarnished or partially hidden, or if the brightness of the reflecting surface is not homogenous);        the image of the marker should be large enough to cover at least one pixel (in practice, at least four pixels); and        the marker should be bright enough to be distinguishable from background noise.        
All of these assumptions and restrictions incur disadvantages for conventional tracking systems. Attempts have been made to solve these problems by using image generating devices with ever higher resolutions. However, very high resolution image generating devices are also very expensive and, therefore, significantly increase costs. Also, some of the above problems cannot be solved simply by using exceedingly high resolution image generating devices. For example, when image points are not completely visible, e.g., in the case of tarnished markers, higher resolution image generating devices offer little or no benefit. Another disadvantage is that high resolution sensors usually have a significantly smaller pixel area and, therefore, are less sensitive. The lower sensitivity of such sensors can result in potentially long exposure times and, therefore, are problematic when objects are moved.
The developers of tracking systems thus find themselves in a sort of “Catch 22”; on the one hand, the markers should not be arbitrarily enlarged, and on the other hand, the resolution should not be arbitrarily increased.